This invention relates to a method and apparatus for forming an extrudate body. The invention relates more particularly to an improved method and apparatus for controlling an average dimensional characteristic of an extrudate body.
In an extrusion method for fabricating polymer plastic bodies of differing profiles or cross sectional configurations, an extrudate stock material in plasticized or liquid form is supplied to and forced through a die having an orifice of desired cross sectional configuration and dimensions. An extrudate output from the die comprises a solid, elongated body having the desired profile or a tubular body having cross sectional configuration. The extrudate may be a sheet, a rod, a tube, or other more complex configuration.
In various applications, it is desirable to maintain the cross sectional dimensional characteritics of the extrudate within a predetermined range of values. The dimensional characteristics which are principally defined by the size of the orifice are, however, subject to variation with temperature, pressure, etc. In order to sense profile dimensional variations, a measuring means has been provided for measuring a dimensional characteristic of the extrudate and for adjusting the process to maintain the dimensional characteristic within a desired range of values. In an exemplary process, the average wall thickness of an extruded plastic pipe or tubing is measured by a nuclear gauge of the back scatter type, or, alternatively by an ultrasonic gauge. These devices are adapted to measure the wall thickness at the location of the gauge. However, since the wall thickness is susceptible to variation about the periphery of the tube, a measurement at a single, peripheral location is not determinative of uniformity in wall thickness of the tube. An average wall thickness has been measured by rotating the gauge about the tubulation with the use of a rotary scanning mount thereby enabling the measurement of the thickness at a number of locations about the periphery of the tube and enabling the calculation of the average thickness of the tube.
Although the described exemplary measuring arrangement is adequate with relatively large diameter tubing, such as tubing having an outside diameter of 3/4 of an inch or greater, and with other forms of configurations, the dimensional resolutions of the aforementioned measuring gauges are unsatisfactory when it is desired to determine the average tubulation thickness for tubing of relatively smaller outside diameter. With these small diameters, the effective spot size of the nuclear gauge is too large and focusing and sighting with an ultrasonic gauge is relatively difficult.
Accordingly, it is an object of this invention to provide an improved method and apparatus for the fabrication of a polymer plastic extrudate.
Another object of the invention is to provide an improved method and apparatus for determining and controlling an average cross sectional dimensional characteristic of an extrudate.
A further object of the invention is to provide an improved method and apparatus for the indirect measurement of an average cross sectional dimensional characteristic of an extrudate.